Streptococci are catalase negative gram positive cocci. They may be classified by the type of hemolysis exhibited on blood agar, by the serologic detection of carbohydrate antigens, or by certain biochemical reactions. Medically important streptococci include Groups A, B, D, S. pneumoniae and the viridans group of streptococci. Lancefield type A (Group A) Streptococcus pyogenes is an important human pathogen—the cause of streptococcal pharyngitis, impetigo and more severe infections such as bacterernia and necrotizing fascitis. The immunologic sequelae of Group A Streptococcal infections are also important health problems—rheumatic carditis is the most common cause of acquired cardiac disease worldwide and post-streptococcal glomerulonephritis is a cause of hypertension and renal dysfunction. Group B Streptococcus agalactiae are the most common cause of serious bacterial infections in newborns, and important pathogens in pregnant women and nonpregnant adults with underlying medical problems such as diabetes and cardiovascular disease. Group D streptococci include the enterococci (Streptococcus faecalis and faecium) and the “nonenterococcal” Group D streptococci. Streptococcus pneumoniae (pneumococcus) is not classified by group in the Lancefield system. Pneumococci are extremely important human pathogens, the most common cause of bacterial pneumonia, middle ear infections and meningitis beyond the newborn period. The viridans group of streptococci include S. milleri, S. mitis, S. sanguis and others. They cause bacteremia, endocarditis, and dental infections. Enterococci are important causes of urinary tract infections, bacteremia and wound infections (predominantly as nosocomial infections in hospitalized patients), and endocarditis. Over the past decade enterococci have developed resistance to many conventional antibiotics and there are some strains resistant to all known antibiotics.
Group B streptococci (GBS) are the most common cause of serious bacterial disease in neonates, and are important pathogens in pregnant women and adults with underlying illnesses (Baker C J. (2000) “Group B streptococcal infections” in Streptococcal infections. Clinical aspects, microbiology, and molecular pathogenesis. (D. L. Stevens and E. L. Kaplan), New York: Oxford University Press, 222-237). Common manifestations of these infections include bacteremia, pneumonia, meningitis, endocarditis, and osteoarticular infections (Baker C J. (2000) “Group B streptococcal infections” in Streptococcal infections. Clinical aspects, microbiology, and molecular pathogenesis. (D. L. Stevens and E. L. Kaplan), New York: Oxford University Press, 222-237; Blumberg H. M. et al. (1996) J Infect Dis 173:365-373). The incidence of invasive GBS disease is approximately 2.6 in 1000 live births and 7.7 in 100,000 in the overall population, with mortality rates that vary from 6 to 30% (Baker C J. (2000) “Group B streptococcal infections” in Streptococcal infections. Clinical aspects, microbiology, and molecular pathogenesis. (D. L. Stevens and E. L. Kaplan), New York: Oxford University Press, 222-237; Blumberg H. M. et al. (1996) J Infect Dis 173:365-373). Although much neonatal disease is preventable by administration of prophylactic antibiotics to women in labor, antibiotic prophylaxis programs can be inefficient, suffer from poor compliance, or fail if antibiotic resistance emerges. No effective prophylaxis strategy for adult infections has been established.
During childbirth, GBS can pass from the mother to the newborn. By one estimate, up to 30% of pregnant women carry GBS at least temporarily in the vagina or rectum without symptoms. Infants born to these women become colonized with GBS during delivery (Baker, C. J. and Edwards, M. S. (1995) “Group B Streptococcal Infections” in Infectious Disease of the Fetus and Newborn Infant (J. S. Remington and J. O Klein), 980-1054). Aspiration of infected amniotic fluid or vaginal secretions allow GBS to gain access to the lungs. Adhesion to, and invasion of, respiratory epithelium and endothelium appear to be critical factors in early onset neonatal infection. (Baker, C. J. and Edwards, M. S. (1995) “Group B Streptococcal Infections” in Infectious Disease of the Fetus and Newborn Infant (J. S. Remington and J. O Klein), 980-1054; Rubens, C. E. et al. (1991) J Inf Dis 164:320-330). Subsequent steps in infection, such as blood stream invasion and the establishment of metastatic local infections have not been clarified. The pathogenesis of neonatal infection occurring after the first week of life is also not well understood. Gastrointestinal colonization may be more important than a respiratory focus in late onset neonatal disease (Baker, C. J. and Edwards, M. S. (1995) “Group B Streptococcal Infections” in Infectious Disease of the Fetus and Newborn Infant (J. S. Remington and J. O Klein), 980-1054). Considerable evidence suggests that invasion of brain microvascular endothelial cells by GBS is the initial step in the pathogenesis of meningitis. GBS are able to invade human brain microvascular endothelial cells and type III GBS, which are responsible for the majority of meningitis, accomplish this 2-6 times more efficiently than other serotypes (Nizet, V. et al. (1997) Infect Immun 65:5074-5081).
Because GBS is widely distributed among the population and is an important pathogen in newborns, pregnant women are commonly tested for GBS at 35-37 weeks of pregnancy. Much of GBS neonatal disease is preventable by administration of prophylactic antibiotics during labor to women who test positive or display known risk factors. However, these antibiotics programs do not prevent all GBS disease. The programs are deficient for a number of reasons. First, the programs can be inefficient. Second, it is difficult to ensure that all healthcare providers and patients comply with the testing and treatment. And finally, if new serotypes or antibiotic resistance emerges, the antibiotic programs may fail altogether. Currently available tests for GBS are inefficient. These tests may provide false negatives. Furthermore, the tests are not specific to virulent strains of GBS. Thus, antibiotic treatment may be given unnecessarily and add to the problem of antibiotic resistance. Although a vaccine would be advantageous, none are yet commercially available.
Traditionally, GBS are divided into 9 serotypes according to the immunologic reactivity of the polysaccharide capsule (Baker C J. (2000) “Group B streptococcal infections” in Streptococcal infections. Clinical aspects, microbiology, and molecular pathogenesis. (D. L. Stevens and E. L. Kaplan), New York: Oxford University Press, 222-237; Blumberg H. M. et al. (1996) J Infect Dis 173:365-373; Kogan, G. et al. (1996) J Biol Chem 271:8786-8790). Serotype III GBS are particularly important in human neonates, causing 60-70% of all infections and almost all meningitis (Baker C J. (2000) “Group B streptococcal infections” in Streptococcal infections Clinical aspects, microbiology, and molecular pathogenesis. (D. L. Stevens and E. L. Kaplan), New York: Oxford University Press, 222-237). Type III GBS can be subdivided into three groups of related strains based on the analysis of restriction digest patterns (RDPs) produced by digestion of chromosomal DNA with Hind III and Sse8387. (I. Y. Nagano et al. (1991) J Med Micro 35:297-303; S. Takahashi et al. (1998) J Inf Dis 177:1116-1119).
Over 90% of invasive type III GBS neonatal disease in Tokyo, Japan and in Salt Lake City, Utah is caused by bacteria from one of three RDP types, termed RDP type III-3, while RDP type III-2 are significantly more likely to be isolated from vagina than from blood or CSF. These results suggest that this genetically-related cluster of type III-3 GBS are more virulent than III-2 strains and could be responsible for the majority of invasive type III disease globally.
Preliminary vaccines for GBS used unconjugated purified polysaccaride. GBS poly- and oligosaccharides are poorly immunogenic and fail to elicit significant memory and booster responses. Baker et al immunized 40 pregnant women with purified serotype III capsular polysaccharide (Baker, C. J. et al. (1998) New Engl J of Med 319:1180-1185). Overall, only 57% of women with low levels of specific antibody responded to the vaccine. The poor immunogenicity of purified polysaccharide antigen was further demonstrated in a study in which thirty adult volunteers were immunized with a tetravalent vaccine composed of purified polysaccharide from serotypes Ia, Ib, II, and III (Kotloff, K. L. et al. (1996) Vaccine 14:446-450). Although safe, this vaccine was only modestly immunogenic, with only 13% of subjects responding to type Ib, 17% to type II, 33% responding to type Ia, and 70% responding to type III polysaccharide. The poor immunogenicity of polysaccharide antigens prompted efforts to develop polysaccharide conjugate vaccines, whereby these poly- or oligosaccharides are conjugated to protein carriers. Ninety percent of healthy adult women immunized with a type III polysaccharide-tetanus toxoid conjugate vaccine responded with a 4-fold rise in antibody concentration, compared to 50% immunized with plain polysaccharide (Kasper, D. L. et al (1996) J of Clin Invest 98:2308-2314). A type Ia/Ib polysaccharide-tetanus toxoid conjugate vaccine was similarly more immunogenic in healthy adults than plain polysaccharide (Baker, C. J. et al (1999) J Infect Dis 179:142-150).
The disadvantage of polysaccharide-protein conjugate vaccines is that the process of purifying and conjugating polysaccharides is difficult, time-consuming and expensive. A protein antigen which could be cheaply and easily produced would be an improvement.
If one were to make a polysaccharide-protein conjugate vaccine, a GBS-specific carrier protein may be preferable to one of the commonly used carriers such as tetanus or diphtheria toxoids because of the potential problems associated with some of these carrier proteins, particularly variable immunogenicity and the problems associated with repeated vaccination with the same carrier protein. Selection of appropriate carrier proteins is important for the development of polysaccharide-protein vaccine formulations. For example, Haemophilus influenzae type b poly- or oligosaccharide conjugated to different protein carriers has variable immunogenicity and elicits antibody with varying avidity (Decker, M. D. et al (1992) J Pediatrics 120:184-189; Schlesinger, Y. (1992) JAMA 267:1489-1494). Repeated immunization with the same carrier protein may also suppress immune responses by competition for specific B cells (epitopic suppression) or other mechanisms. This is of particular concern for the development of GBS vaccines since recently developed poly/oligosaccharide-protein conjugate vaccines against the bacteria H. influenzae, S. pneumoniae, and N. meningitidis all utilize a restricted number of carrier proteins (tetanus toxoid, CRM197, diptheria toxoid), increasing the number of exposures to these carriers an individual is likely to receive. Additionally, using tetanus as a carrier protein offers no specific advantage beyond the improved immunogenicity of the vaccine. A second-generation vaccine containing a GBS-specific carrier protein would enhance immunogenicity and have an advantage in that a GBS-specific immune response would be generated against both the carrier protein and the poly/oligosaccharide.
Therefore, in view of the aforementioned deficiencies attendant with prior art vaccines and methods, it should be apparent that there still exists a need in the art for an effective and immunogenic GBS vaccine. The availability and use of a GBS polypeptide in a conjugate vaccine is desirable. A GBS polypeptide which is present or expressed in all GBS serotypes would have the added advantage of providing broad, general immunity across many GBS serotypes. It would be particularly relevant and useful to provide a streptococcal vaccine or immunogen which is expressed broadly in various streptococcal species, whereby broad or general immunity against multiple and unique groups of streptococci (for instance, Group A, Group B and S. pneumoniae), particularly against distinct virulent and clinically relevant streptococcal bacteria, could thereby be generated.
The citation of references herein shall not be construed as an admission that such is prior art to the present invention.